Commercial SnO2 and its interface with a-Si have been studied by photoemission. The valence band (V.B.) spectra show that in some regions of SnO2 a large amount of metallic Sn is present. Similar V.B. spectra were obtained in good regions as a consequence of H bombardment. The reduction of SnO2 to metallic Sn is confirmed by Auger and electron energy loss measurements. The V.B. discontinuity at the interface was 2.3 eV and reduced to 1.1 eV by post-hydrogenation, while being zero for reduced SnO2 substrates. Finally, there is evidence of diffusion of O and Sn into silicon.